1. Field of the Invention
The present invention generally relates to data communication and, more particularly, to calibration and compensation of signal impairments introduced by cable interconnects used for data communication.
2. Related Art
Data communication standards are defined to ensure interoperability between communicating devices. Wired-line communication standards may also define requirements and tolerances for data cables through which the devices communicate. For example, a widely used data communication standard for communicating between computers and peripheral devices is Universal Serial Bus (USB). USB signals are transmitted differentially through twisted-pair wires inside data cables. These data cables introduce impairments on the signals due to cable parasitics, resistive loss, impedance mismatch, or other factors. These impairments may include as intra-pair skew of the differential signals, insertion loss, crosstalk, and other types of degradation to the signals. Critically, as data rate goes up, minimizing signal impairments and maintaining signal integrity become even more important because signals become less tolerant to impairments. For example, at the USB 3.0 data rate of 4.8 Gbits/second, minimizing signal impairments is critical to ensure reliable data communication. While the USB 3.0 standard defines strict tolerances on signal impairments for data cables used between devices, the standard is less well defined for internal data cables used inside the devices. As a result, a USB 3.0 device may pass a compliance test, but may fail in the field after the device is connected to an internal data cable due to signal impairments introduced by the internal data cable.
Accordingly, there is a need for techniques to calibrate and compensate for signal impairments introduced by internal cables used in data communication devices, especially high speed data communication devices such as those using USB 3.0.